You have to look at the type of crank and see if the chainring is separate. Most designs do have detachable rings, but certainly not all. Even so, there may not be availability of a smaller ring to fit. The better option may be to use a larger sprocket in back. These are usually less expensive, and easier to swap out (not always). The only drawback of enlarging the rear sprocket is that it'll need a longer chain, so that adds to the expense.
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